This invention relates to a high-speed electromagnetic actuator, and particularly to a fuel injector for an internal combustion engine. More particularly, this disclosure relates to an apparatus and method of actuating a fuel injector or similar metering device by magnetostrictive transduction.
A conventional method of actuating a fuel injector is by use of an electromagnetic solenoid arrangement. A solenoid is an insulated conducting wire wound to form a tight helical coil. When current passes through the wire, a magnetic field is generated within the coil in a direction parallel to the axis of the coil. When the coil is energized, the resulting magnetic field exerts a force on a moveable ferromagnetic armature located within the coil, thereby causing the armature to move a needle valve into an open position in opposition to a force generated by a return spring. The force exerted on the armature is proportional to the strength of the magnetic field; the strength of the magnetic field depends on the number of turns of the coil and the amount of current passing through the coil.
In the conventional fuel injector, the point at which the armature, and therefore the needle, begins to move varies primarily with the spring preload holding the injector closed, the friction and inertia of the needle, fuel pressure, eddy currents in the magnetic materials, and the magnetic characteristics of the design, e.g., the ability to direct flux into the working gap. Generally, the armature will not move until the magnetic force builds to a level high enough to overcome the opposing forces. Likewise, the needle will not return to a closed position until the magnetic force decays to a low enough level for the closing spring to overcome the fuel flow pressure and needle inertia. In a conventional injector design, once the needle begins opening or closing, it may continue to accelerate until it impacts with its respective end-stop, creating wear in the needle valve seat, needle bounce, and unwanted vibrations and noise problems. Accordingly, a need exists for an improved fuel injector actuation method that will provide reduced noise, longer seat life, elimination of bounce, and full actuator force applied during the entire armature stroke, where the force is large as compared with the force resulting from fuel pressure effects.
A second less conventional method of actuating a fuel injector is by use of a piezoelectric actuator consisting of a stack of piezoceramic or piezocrystal wafers bonded together to form a piezostack transducer. Transducers convert energy from one form to another and the act of conversion is referred to as transduction. Piezoelectric transducers convert energy in an electric field into a mechanical strain in the piezoelectric material. The piezostack may be attached to the mechanical member or needle performing a similar function as the needle in the conventional injector. When the piezostack has a high voltage potential applied across the wafers, the piezoelectric effect causes the stack to change dimension, thereby opening the fuel injector. An advantage of piezoelectric actuation is that the ultrasonic operation results in improved fuel atomization. Another advantage of piezoelectric actuation is that the piezostack applies full force during the armature travel, allowing for controlled trajectory operation.
However, as can be observed from the following derivation, because the magnetic energy density is several orders of magnitude greater than the electric energy density, it is necessary to operate piezoelectric actuators at very high voltages, e.g. on the order of 200 volts. In addition, a piezoelectric actuator requires a complex high voltage driver with the capacity to slew hundreds of volts rapidly into a capacitive load while maintaining high voltage isolation.
Comparing the energy densities in magnetic and electric fields, it can be shown that the energy density in a magnetic field is given by:
B2/2xcexc, where xcexc=permeability of free space=12.57xc3x9710xe2x88x927 Tm/A;
the energy density in an electric field is given by:
xcex5E2/2, where xcex5=permittivity of free space=8.85xc3x9710xe2x88x9212 C/Nm2; and
xe2x80x83thus the ratio of magnetic to electric energy densities=B2/xcexcxcex5E2.
Conservatively, the magnetic energy density is several orders of magnitude greater than the electric energy density, given that most ferromagnetic materials saturate above 1Tesla (usually around 2 Tesla) and most dielectrics break down at above 100,000 Volts per mm (usually higher).
Accordingly, because magnetic energy density is several orders of magnitude greater than the electric energy density, piezoelectric (i.e., electrostrictive) transduction requires high voltages to generate a useful electric energy density and hence a useful strain in the piezoelectric material. Thus, a need exists for a fuel injector capable of operating on the magnetic equivalent of the piezoelectric effect, i.e., magnetostriction.
The term xe2x80x9cmagnetostrictionxe2x80x9d literally means magnetic contraction, but is generally understood to encompass the following similar effects associated with ferromagnetic materials: the Guillemin Effect, which is the tendency of a bent ferromagnetic rod to straighten in a longitudinal magnetic field; the Wiedemann Effect, which is the twisting of a rod carrying an electric current when placed in a magnetic field; the Joule Effect, which is a gradual increasing of length of a ferromagnetic rod when subjected to a gradual increasing longitudinal magnetic field; and the Villari Effect, which is a change of magnetic induction in the presence of a longitudinal magnetic field (Inverse Joule Effect).
The dimensional changes that occur when a ferromagnetic material is placed in a magnetic field are normally considered undesirable effects because of the need for dimensional stability in precision electromagnetic devices. Therefore, manufacturers of ferromagnetic alloys often formulate their alloys to exhibit very low magnetostriction effects. All ferromagnetic materials exhibit magnetic characteristics because of their ability to align magnetic domains. As shown in FIG. 1, strongly magnetostrictive materials characteristically have domains that are longer in the direction of their polarization (North/South) and narrower in a direction perpendicular to their polarization, thus allowing the domains to change the major dimensions of the ferromagnetic material when the domains rotate.
For example, the magnetostrictive alloy Terfenol-D (Tb0.3Dy0.7Fe1.9), is capable of approximately 10 xcexcm displacements for every 1 cm of length exposed to an approximately 500 Oersted magnetizing field. The general equation for magnetizing force, H, in Ampere-Turns per meter (1 Oersted=79.6 AT/m) is:
H=IN/L, where I=Amperes of current; N=number of turns; and L=path length.
Terfenol-D is often referred to as a xe2x80x9csmart materialxe2x80x9d because of its ability to respond to its environment and exhibit giant magnetostrictive properties. The present invention will be described primarily with reference to Terfenol-D as a preferred magnetostrictive material. However, it will be appreciated by those skilled in the art that other alloys having similar magnetostrictive properties may be substituted and are included within the scope of the present invention.
A magnetostrictively actuated fuel injector is provided. The fuel injector has a body having a cavity along the longitudinal axis, an inlet port, an outlet port having a valve seat and a fuel passageway extending from the inlet port to the outlet port. A magnetostrictive element having a predetermined length is disposed in the cavity and is in operative contact with a needle having a tip proximate the valve seat forming a valve. A coil is provided for generating a magnetic field. The coil is disposed proximate the magnetostrictive element such that magnetic flux passes through the magnetostrictive element upon excitation of the coil, causing the predetermined length to increase, thereby actuating the valve